The invention relates generally to the field of cardiac pacers, and more particularly to sensing systems and methods for sensing minute ventilation to detect a patient's metabolic demand as an input for demand rate-variable cardiac pacers.
It is known that some patients suffering from cardiovascular illness cannot generate nerve signals to cause the cardiac muscle contractions which pump blood throughout their bodies. Artificial pacers have been developed to transmit signals to the patients' cardiac muscles and substitute for natural signals.
Changes in a patient's metabolic rate, caused for example by varying physical activity, create varying body demands for cardiovascular pumping. In response to the need for demand rate-variable pacers, different methods have been developed for sensing metabolic demands and adjusting the signal output of the pacer to the patient's heart. Variables which have been, or are being, studied as indicators of metabolic rate response in pacing include: venous oxygen saturation; venous Ph; stoke volume; right ventricular temperature; right ventricular pressure; QT interval; physical activity (i.e., gross body movement rates); and respiratory rate.
Of these variables, respiratory rate, and more specifically minute ventilation (defined as the product of the patient's respiration rate and the tidal volume of each breath, i.e. the volume of air breathed per minute), appears to be the most accurate measure of metabolic demand, since it is a parameter actually calculated by the patient's brain to set the heart pumping rate. A pacer having a sensing system designed to detect the patient's minute ventilation rate and to vary its own electrical pacing pulse rate in response thereto would desirably mimic the natural human body function.
One pacer apparatus for sensing ventilation rate disclosed in U.S. Pat. No. 4,567,892 involves the subcutaneous implantation of a pair of leads in the chest wall and measurement of changes in electrode impedance during respiration. Such sensing is effected by driving a pulsed current through a pair of leads and measuring with electrical circuitry the voltage drop of the current impulses to determine changes in electrical impedance. The drawback of the system disclosed in U.S. Pat. No. 4,567,892 is that an extra pair of leads must be implanted in the patient in addition to the normal pacing leads. Pacing leads may be installed transvenously into the patient's cardiovascular system, such as by inserting an atrial J-electrode in the patient's right atrium and it would also be desirable to implant ventilation sensors transvenously while implanting pacer leads.
It is an object of the present invention to sense the minute ventilation rate of a patient for use by a demand rate-variable pacer without installation of additional sensing leads.
It is another object of the invention to provide a minute ventilation sensing system and method for sensing minute ventilation which may be easily implanted by surgeons using known techniques.
If is a further object of the invention to create a minute ventilation sensing system and a method for sensing minute ventilation which may be implanted transvenously within the patient's cardiovascular system.
Lastly, it is an object of the invention to construct a minute ventilation sensing system and method for sensing minute ventilation which may be combined with known types of pacer leads, such as atrial J-electrodes, and may be implanted simultaneously with the other leads.